Linkage and Crossing Over of Genes (With Diagram) | Genetics

In this article we will discuss about the linkage and crossing over of genes.

In Mendelian experiment the common sweet peas with two or more “elements” (now called genes) are considered heterozy­gous. In a hybrid, the segregation of any one is completely independent of the rest. That means, all the genes are distributed to the gametes at random.

Mendel considered inheritance of seven characters and all the characters showed random assortment. It was later demon­strated cytologically that peas have seven pairs of chromosomes. It was genetically shown that the genes for the characters studied by Mendel are each located in a different member of the seven pairs of chromosomes.

The independent assortment between two gene pairs, each with complete dominance and affecting different charac­ters, results in 9: 3: 3: 1 ratio .among the offspring of heterozygotes for both pairs (AaBb x AaBb).

But Bateson and Punnet in England in 1906 found exception to this law when they bred a di-hybrid strain of sweet pea. They crossed purple (P) flowered-long pollen grained (L) sweet peas with red flowered (p) round pollen grained (1) ones.

The f₁ gen­eration was purple flowered-long pollen grained (P-L-). But in f₂ generation purple-long, purple- round, red-long and red-round appeared in the ratio of 11: 1: 1: 3 with too many parental types.

Bateson and Punnet explained that the gametic coupling between P and L and p and 1 lead to such high ratio of parental combination. Again, in the next generation, Pl and pL characters tend to come together more often. This is called repulsion where the parental associations did not form but new combinations tend to persist.

Both coupling and repulsion are possible when genes are arranged linearly along the length of chromosomes. When two genes remain in the same chromosome they tend to be inherited together to the offspring. This is called linkage and two genes are linked.

But, during reduction division, ex­change of segments of a pair of chromo­somes occur, as a result linked genes are separated and recombination occurs. This is called crossing over (Fig. 46.3). T. H. Mor­gan (1911) showed in his experiment with Drosophila, the linear arrangement of genes in a chromosome and gave explanation of both coupling and repulsion.

So linkage may be defined as the ten­dency of two or more genes in the same chromosome to remain in their original combination in the process of inheritance.

Thus, (i) The linked genes remain in the same chromosome and tend to be transmitted together generation after generation, (ii) Linked genes never show Mendel’s Principle of Independent Assortment, (iii) Less dis­tance between the genes show stronger link­age bond. 

Example of Linkage:

In Drosophila, grey body (b⁺) and long wing (v⁺) characters are dominant over black body (b) and vestigeal wing (v). When a pure grey-long is crossed to a black vestigeal the offspring produced in fi generation are grey-long hybrid (Fig. 46.4).

In the f₁ female genotype, the chromo­some might come together and separate without crossing over between b⁺ and v⁺ giving eggs of two non-crossover classes b⁺ v⁺ and b v. The second possibility is that crossing over might take place between b⁺ and v⁺ given eggs of two crossover classes b⁺ v and b v⁺

If a cross is made between fi female and a black-vestigeal male (double recessive) four categories of offspring’s will be pro­duced (Fig. 46.4). The grey-long and black-vestigeal are the no crossovers (parental combination) and constitute more than 50% of the total offspring’s while the grey-vestigeal and black-long are crossover classes (new combination) and constitute less than 50%. So the tendency of two genes to remain in their original combination is always greater. In this case, as the crossing over breaks the linkage in certain.percentage, it is called incomplete linkage.

Complete Linkage:

The complete linkage or absence of cross­ing over between the genes on the same chromosome is a rarity in most sexually reproducing species. However, in male Drosophila, complete linkage is observed.

In other cases, when genes are so closely as­sociated that they are always transmitted together upon coming from the same parent, linkage between them is considered com­plete. The 4th chromosome mutations of fruit fly Drosophila melanogaster show little or no independent assortment during transmis­sion.

A Drosophila with 4th chromosome caring genes producing bent wings and shaven bristles crossed to a normal fly produce normal appearing heterozygous in f₁ germination. When such f₁ flies are crossed to the homozygous bent-shaven   stock, the offspring’s are almost all phenotypically either bent and shaven or normal for both characters. The complete absence of new combination is the evidence of very strong and complete linkage between the two genes on the same homologue. 

Test Cross:

The cross of fi hybrids with a double recessive male or female is known as test cross or back cross. In this process, only recessive genes will be added so that all the mutations will be expressive. It is the sim­plest way to determine the per cent of crossing over.